Biomedical subjects

Joel L Sachs

Publications and source records attributed to Joel L Sachs.

4 recordsLinked to original sources

PubMed2006

Pathways to mutualism breakdown.

Mutualisms are ubiquitous in nature despite the widely held view that they are unstable interactions. Models predict that mutualists might often evolve into parasites, can abandon their partners to live autonomously and are also vulnerable to extinction. Yet a basic empirical question, whether mutualisms commonly break down, has been mostly overlooked. As we discuss here, recent progress in molecular systematics helps address this question. Newly constructed phylogenies reveal that parasites as well as autonomous (non-mutualist) taxa are nested within ancestrally mutualistic clades. Although models have focused on the propensity of mutualism to become parasitic, such shifts appear relatively rarely. By contrast, diverse systems exhibit reversions to autonomy, and this might be a common and unexplored endpoint to mutualism.

Animals↗

PubMed2006

A shift to parasitism in the jellyfish symbiont Symbiodinium microadriaticum.

One of the outstanding and poorly understood examples of cooperation between species is found in corals, hydras and jellyfish that form symbioses with algae. These mutualistic algae are mostly acquired infectiously from the seawater and, according to models of virulence evolution, should be selected to parasitize their hosts. We altered algal transmission between jellyfish hosts in the laboratory to examine the potential for virulence evolution in this widespread symbiosis. In one experimental treatment, vertical transmission of algae (parent to offspring) selected for symbiont cooperation, because symbiont fitness was tied to host reproduction. In the other treatment, horizontal transmission (infectious spread) decoupled symbiont fitness from the host, potentially allowing parasitic symbionts to spread. Fitness estimates revealed a striking shift to parasitism in the horizontal treatment. The horizontally transmitted algae proliferated faster within hosts and had higher dispersal rates from hosts compared to the vertical treatment, while reducing host reproduction and growth. However, a trade-off was detected between harm caused to hosts and symbiont fitness. Virulence trade-offs have been modelled for pathogens and may be critical in stabilising 'infectious' symbioses. Our results demonstrate the dynamic nature of this symbiosis and illustrate the potential ease with which beneficial symbionts can evolve into parasites.

Animals↗

PubMed2005

Experimental evolution of conflict mediation between genomes.

Transitions to new levels of biological complexity often require cooperation among component individuals, but individual selection among those components may favor a selfishness that thwarts the evolution of cooperation. Biological systems with elements of cooperation and conflict are especially challenging to understand because the very direction of evolution is indeterminate and cannot be predicted without knowing which types of selfish mutations and interactions can arise. Here, we investigated the evolution of two bacteriophages (f1 and IKe) experimentally forced to obey a life cycle with elements of cooperation and conflict, whose outcome could have ranged from extinction of the population (due to selection of selfish elements) to extreme cooperation. Our results show the de novo evolution of a conflict mediation system that facilitates cooperation. Specifically, the two phages evolved to copackage their genomes into one protein coat, ensuring cotransmission with each other and virtually eliminating conflict. Thereafter, IKe evolved such extreme genome reduction that it lost the ability to make its own virions independent of f1. Our results parallel a variety of conflict mediation mechanisms existing in nature: evolution of reduced genomes in symbionts, cotransmission of partners, and obligate coexistence between cooperating species.

Bacteriophages↗

PubMed2004

The evolution of cooperation.

Darwin recognized that natural selection could not favor a trait in one species solely for the benefit of another species. The modern, selfish-gene view of the world suggests that cooperation between individuals, whether of the same species or different species, should be especially vulnerable to the evolution of noncooperators. Yet, cooperation is prevalent in nature both within and between species. What special circumstances or mechanisms thus favor cooperation? Currently, evolutionary biology offers a set of disparate explanations, and a general framework for this breadth of models has not emerged. Here, we offer a tripartite structure that links previously disconnected views of cooperation. We distinguish three general models by which cooperation can evolve and be maintained: (i) directed reciprocation--cooperation with individuals who give in return; (ii) shared genes--cooperation with relatives (e.g., kin selection); and (iii) byproduct benefits--cooperation as an incidental consequence of selfish action. Each general model is further subdivided. Several renowned examples of cooperation that have lacked explanation until recently--plant-rhizobium symbioses and bacteria-squid light organs--fit squarely within this framework. Natural systems of cooperation often involve more than one model, and a fruitful direction for future research is to understand how these models interact to maintain cooperation in the long term.

Algorithms↗